The present invention relates to a process and an apparatus for decontaminating exhaust gas from a fusion reactor fuel cycle of exhaust gas components containing chemically bonded tritium and/or deuterium, in which tritium and/or deuterium is/are liberated from its/their compounds, separated out from the exhaust gas and fed back into the fuel cycle.
The exhaust gas from a fusion reactor fuel cycle contains about 85%-by-volume noble gases and about 15%-by-volume impurities, including small residual amounts of heavy hydrogen. The impurities accumulate in the form of argon, "tritiated" and/or "deuterated" hydrocarbons, particularly tritiated and/or deuterated" CH.sub.4, tritiated and/or deuterated water and tritiated and/or deuterated ammonia. The exhaust gas must thus be liberated both from free tritium as well as impurities containing tritium and be decontaminated to a value level permissible for emission before the exhaust gas remainder can be released into the surrounding atmosphere. Moreover, it is desirable to recover the tritium and deuterium from their compounds and to feed the tritium and deuterium back into the fuel cycle, not least because it is in this way guaranteed that the tritium is kept from entering the surrounding atmosphere.
A process and an apparatus for decontaminating exhaust gas of tritium and/or deuterium has been suggested by Kerr et al, "Fuel Cleanup System for the Tritium Systems Test Assembly: Design and Experiments", Proceedings of Tritium Technology in Fission, Fusion and Isotopic Application, Dayton, Ohio, Apr. 29, 1980, at pages 115 to 118. According to one process described by Kerr et al, the exhaust gas containing the impurities is first passed through an intermediate container, that is, a variable volume surge tank which is used to remove flow fluctuations and provide a constant feed pressure. The exhaust gas is then passed to a first catalytic reactor in which any free oxygen is reduced and combined with hydrogen at 450.degree. K. to form water. The exhaust gas is then sent to a molecular sieve bed at 75.degree. K. in which all impurities are adsorptively removed and are thus separated out from the exhaust gas. When the capacity of the molecular sieve bed is exhausted, it is heated to 400.degree. K. to desorb the impurities which are then sent to a second catalytic reactor in the form of an oxygen-supplying packed bed operating at 800.degree. K. where the impurities (e.g., ammonia and hydrocarbons) are oxidized into tritium- and/or deuterium-containing water and into tritium- and/or deuterium-free compounds, namely into CO.sub.2, N.sub.2 and Ar. The tritium- and/or deuterium-containing water then is frozen out at 160.degree. K., and thereafter the frozen water is periodically vaporized. The vapors are fed into a hot uranium metal bed which acts as a getter and which at 750.degree. K. transforms (reduces) the water into D- and/or T-containing hydrogen and stable UO.sub.2. In lieu of the reduction by means of the uranium metal bed, Kerr et al state that the reduction can also be carried out with the aid of an electrolytic cell when such a cell becomes available.
Kerr et al also describe a process based on hot uranium metal getters. In this process, the exhaust gas, after leaving the variable volume surge tank, enters a primary uranium bed operating at 1170.degree. K. In this bed, impurities are removed by chemical reactions that form uranium oxides, carbides, and nitrides. The inert argon, with traces of the other impurities, passes through the primary uranium bed and is sent to a molecular sieve bed as in the above-described process. The regenerated argon, with a small amount of tritium, is sent from the molecular sieve bed to a titanium bed, at 500.degree. K., which collects DT and passes on an argon stream containing only tenths of a ppm of DT. Kerr et al state that a disadvantage of this process is that operating temperatures of 1170.degree. K. cause permeation and material problems.
Kerr et al also describe the use of palladium diffusers, and state that they have numerous disadvantages including the need for elevated pressures, reported brittle failures during temperature cycling, reported poisoning by ammonia and methane, and the fact that they can not produce an impurity stream free of hydrogen isotopes.
The processes suggested heretofore for decontaminating exhaust gases of tritium and/or deuterium have the following disadvantages: many steps in the process; high temperature and thus the danger of tritium losses through permeation; operation of the oxygen-supplying packed bed (second catalytic reactor) at high temperatures, with which is associated a possible sintering of the packed bed particles as well as an excess of oxygen given off (deactivation), which strains the hot metal getter; transformation of ammonia and hydrocarbons by oxidation at the second catalytic reactor with formation of water and subsequent reduction of the water created by the hot metal getter (strain on the hot metal getter); oxidation of hydrogen to water at the first catalytic reactor and subsequent reduction of the water created by the hot metal getter (strain of the hot metal getter); high radioactive waste solids and creation of nitrogen oxides during NH.sub.3 oxidation of the oxygen-supplying packed solids bed (second catalytic reactor).